Graphics Card: http://static.howstuffworks.com/gif/graphics-card-5.jpg
keep in mind you dont need a graphics card if the motherboard has onboard graphics. however, since the onboard graphics thats built into motherboards in very lousy i suggest you use a dedicated graphics card if you anything besides word processing and internet browsing.
ok so i dont think i forgot anything but i might have. below are a few random compnenents that you may come across: